The "main" task as you describe it is the summary of the sub-tasks. When it comes to "Duration" that includes the time between tasks - ie, it's telling you the whole duration of time for that summary task to be completed. Duration for a summary task is measured from the start of the first sub-task to the finish of the last sub-task.
What you want is the "Work" time. Next to the duration column, insert a new column and select the "Work" field. This describes the amount of time required to complete the task. Assign a resource to the task, then look at the values against each task & the summary.
You will find a task has a duration (for example) of 1 day, work of 1 day (8 hours). The summary task duration will still say 7 days (which is correct) but the work to be done over those 7 days is 3 days.
